This invention relates in general to a method of repositioning teeth for use in orthodontic treatment. Particularly, this invention relates to the use of orthodontic appliances for producing tooth movements. More particularly, this invention relates to the use of a plurality of elastic repositioning appliances for producing such tooth movements.
Orthodontic treatment is based on the principle that if prolonged pressure is applied to a tooth, tooth movement will occur as the bone around the tooth remodels. Bone is selectively removed in some areas and added in others. In essence, the tooth moves through the bone as it remodels, carrying its attachment apparatus with it as the socket of the tooth migrates. This attachment apparatus is a heavy collagenous supporting structure called the periodontal ligament (PDL) which attaches the tooth to the adjacent alveolar bone. The tooth remodeling is mediated by the PDL. Although the PDL is adapted to resist forces of short duration, prolonged force produces a different physiologic response, namely remodeling of the adjacent bone. Orthodontic tooth movement is made possible by the application of prolonged forces.
The simplest form of orthodontic movement is tipping. Tipping movements are produced when a single force is applied against the crown of a tooth. When this is done, the tooth rotates around its xe2x80x9ccenter of resistancexe2x80x9d, a point at which resistance to movement can be concentrated for mathematical analysis. The center of resistance for a tooth is at the approximate midpoint of the embedded portion of the root, about halfway down the root. When the tooth rotates in this fashion, the PDL is compressed near the root apex on the same side as the force and at the crest of the alveolar bone on the opposite side. These areas account for only one-half the PDL area that could actually be loaded. Therefore, pressure in the two areas where it is concentrated is high in relation to the force applied to the crown. For this reason, forces used to tip teeth may be quite low and such movement may be easily achieved.
In many cases, mere tipping of the teeth is insufficient in completing orthodontic treatment. Translation, bodily movement of the tooth where the root apex and crown move in the same direction by the same amount, is often necessary. This may be accomplished by applying forces to the crown which create counterbalancing moments. Thus, the tooth would remain upright and move bodily. In this case, the total PDL area is loaded uniformly. Therefore, it is apparent that to produce the same pressure in the PDL and the same biologic response, twice as much force would be required for bodily movement as for tipping. To move a tooth so that it is partially tipped and partially translated would require forces intermediate between those needed for pure tipping and pure bodily movement.
For a number of reasons, including lowering the level of force applied to the teeth, at least part of the translation process is often accomplished by xe2x80x9ctippingxe2x80x9d and xe2x80x9cuprighting.xe2x80x9d This technique is often referred to as the Begg approach or technique. Raymond Begg adapted current technology in the 1920""s to produce the Begg appliance. The Begg appliance uses xe2x80x9cstationary anchoragexe2x80x9d, in which movement teeth are allowed to tip while anchor teeth are allowed to only move bodily if at all. In the case of closing a premolar extraction site, anterior teeth would first be tipped distally. As a second step, the tipped teeth would be uprighted, moving the canine roots distally and torquing the incisor roots lingually. Using this technique, the optimum pressure for the anterior segment would be produced by about half as much force as if the anterior teeth were to be retracted bodily.
The Begg appliance and appliances using the Begg technique are typically bonded to the teeth. Bonding facilitates tooth movements by providing consistent points of purchase on the tooth for manipulation. During tipping, the crown is tipped in the direction of tooth movement. Interconnected bonded brackets or bands prevent the crown from substantially intruding the gingival line. The bonded appliances are then utilized to upright the tooth at its destination. However, many removable appliances, such as elastic repositioning appliances, do not have the benefit of bonding to manipulate the teeth. In these cases, teeth are commonly repositioned by manipulation of the crown directly. Points of purchase may be increased with the use of attachment devices on the crown. However, as a tooth becomes intruded during tipping, the portion of exposed surface area on the crown decreases. This diminishes the ability of an appliance, such as an elastic repositioning appliance, to xe2x80x9cgrabxe2x80x9d onto a tooth for further manipulation. Therefore, the ability of accomplishing long, complicated movements in later stages of orthodontic treatment may be hindered. Further, translation of the tooth in the crown-first position is more difficult with the use of elastic repositioning appliances due to the slope of the crown. Force applied to the crown may serve to further intrude the tooth or may be reduced to a level of ineffectivity.
For these reasons, it would be desirable to provide alternative methods and systems for moving teeth. These methods should be compatible for use with any type of orthodontic appliance, both bonded and removably attachable. However, such methods may be particularly suitable for use with removable orthodontic appliances, including elastic repositioning appliances. In the case of elastic repositioning appliances, such methods and systems should also benefit the utilization of attachment devices, if available, and possibly lessen the need for additional attachment devices. At least some of these objectives will be met by the methods and systems of the present invention described hereinafter.
The present invention provides methods and systems of repositioning teeth for use in orthodontic treatment, with particular applicability to removable elastic repositioning appliances. Some traditional methods of repositioning teeth are most suitable for orthodontic appliances which are bonded to the teeth. Such bonding allows manipulation of the teeth which is not as easily affordable with removable appliances. Therefore, the present invention provides alternative repositioning movements which are particularly suited for removable elastic appliances.
Such appliances preferably comprise a polymeric shell having teeth-receiving cavities formed therein. The cavities generally conform to the patient""s teeth, but certain cavities are slightly out of alignment with the initial tooth configuration. The appliance, however, is sufficiently resilient to accommodate or conform to the misaligned teeth, and will apply sufficient resilient force against such misaligned teeth in order to reposition the teeth to the desired position. Repetition of this process with successive appliances comprising new configurations eventually moves the teeth through a series of intermediate configurations to a final desired configuration. A full description of an exemplary elastic polymeric positioning appliance is described in U.S. Pat. No. 5,975,893, and in published PCT application WO 98/58596 which designates the United States and which is assigned to the assignee of the present invention. Both documents are incorporated by reference for all purposes.
Repositioning of teeth in orthodontic treatment typically involves a number of individual tooth movements, including but not limited to tipping, translation, root uprighting, rotation, extrusion and intrusion. Such movements are coordinated to reposition teeth anywhere within a gingival plane. A gingival plane may be defined as a plane formed by the arch of the gingival line. Often, tooth movements, such as translations, are described as occurring along the gingival line. This typically corresponds to mesial-distal movements. However, many labial-lingual movements are also required in orthodontics and occur in a direction substantially perpendicular to the gingival line. Likewise similar movements may be required along lines between these two axes. Therefore, a gingival plane may be defined to incorporate all of these lines or axes along which movement may take place.
In the methods of the present invention, a series of tooth movements are provided for moving a tooth from a first location to a desired location within a gingival plane. The movements may be grouped into three phases: 1) determining that the tooth is positioned so that its longitudinal axis is angled, having the axis portion passing through the root closer to the desired location than the axis portion passing through the crown, 2) translating the tooth toward the desired location with the axis portion passing through the root leading, and 3) positioning the tooth so that the longitudinal axis is substantially perpendicular to the gingival plane at the desired location.
In a first aspect of the methods of the present invention, the first phase of movements may involve a series of individual simultaneous or coordinated movements. In a preferred embodiment, the series may include three such movements: 1) rotation of the tooth around its center of rotation so that its longitudinal axis is angled less than 90 degrees from the gingival plane, 2) translation of the tooth toward the desired location, and 3) extrusion of the tooth so that the crown is not substantially intruded. Such movements are particularly applicable for achievement with an elastic repositioning appliance. Rotational forces applied to the crown of a tooth by an elastic appliance inherently apply simultaneous translational and extrusional forces. For example, rotational forces applied to the crown in the mesial direction will result in translation in the distal direction and extrusion, in addition to rotation. Therefore, it is desired to rotate the tooth so that the portion of the longitudinal axis passing through the root is closer to the desired location than portion passing through the crown, i.e. root-first. However, the tooth may be presented in this position and repositioning may begin with the second phase.
In a second aspect of the methods of the present invention, the second phase of movements involves translating the tooth toward the desired location with the axis portion passing through the root leading. Translation of the tooth in this orientation provides usable points of purchase on the crown through which an elastic positioning appliance may transmit translational forces of sufficient level and control.
In a third aspect of the methods of the present invention, the third phase of movements involves a series of individual simultaneous or coordinated movements. In a preferred embodiment, the series may include three movements: 1) rotation of the tooth around its center of rotation so that its longitudinal axis is angled less than 90 degrees from the gingival plane, 2) translation of the tooth toward the desired location, and 3) intrusion of the tooth so that the crown is not substantially intruded. As in the first phase, such movements are particularly applicable for achievement with an elastic repositioning appliance. When rotational forces are applied to the crown of a root-first tooth by an elastic appliance in the opposite direction as Phase 1, the appliance inherently applies simultaneous translational and intrusional forces. For example, rotational forces applied to the crown in the distal direction will result in translation in the distal direction and intrusion, in addition to rotation. Therefore, such movements will position the tooth at the desired location with the longitudinal axis substantially perpendicular to the gingival plane.
In a fourth aspect, the systems and methods of the present invention involve the use of one or more attachment devices positioned on the teeth for use in transmitting forces from the elastic positioning adjustment appliance to the teeth. The attachment devices may be of simple construction, having an attachment body and a base. The base is typically bonded to the tooth with the body protruding therefrom. This affords an additional point of purchase on the tooth for added leverage and control. When the elastic appliance is positioned on the teeth, the appliance may apply force to the attachment devices for transmission to the underlying teeth. Thus, the attachment devices may be utilized in implementing the above described repositioning movements. A full description of exemplary attachment devices is described, co-pending application Ser. No. 09/454,786, assigned to the assignee of the present invention, incorporated by reference for all purposes.
In a fifth aspect, the methods of the present invention may be used with more conventional orthodontic devices, such as brackets to the teeth. In these cases, the above described movements may be translated to the bonded bracket by the use of wires, elastic bands or similar devices. Likewise, such forces may be transmitted from an appliance to an attachment device with similar wire, elastic bands or devices.
A further understanding of the nature and advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings.